The present invention pertains to the field of medical diagnostic imaging and more particularly to the optimization, both quantitatively and qualitatively, of light output from scintillators used in such imaging.
Many medical imaging devices such as gamma cameras use a scintillation crystal to detect the radiation that is processed to provide images of the interior of patients. Radiation, such as gamma radiation, that has passed through or out of a patient being imaged impacts the scintillator generating scintillations, i.e. flashes of light. Light sensors such as photomultiplier tubes (PMTs) are optically coupled to the scintillation crystal, generally through transparent means such as a glass plate. The flashes of light which are sometimes referred to as xe2x80x9can eventxe2x80x9d, are detected by the PMTs and converted into electrical pulses. The electrical pulses are processed to provide the images. Thus the scintillation generated light is at the heart of the imaging by devices such as gamma cameras.
In practice, when an event occurs, the light from the scintillation crystal strikes and illuminates an area of the glass coupling the scintillation crystal to the PMTs. The location of the event needed to provide an image is computed with algorithms that use the outputs of the PMTs contiguous to illuminated areas. The accuracy of the location determination, which also effects the image uniformity, is maximized when a plurality of PMTs are illuminated by the event. However, for those portions of the image that are over a single PMT, the events will appear to be bunched at the center of the PMT. For those portions of the image that are between PMTs, light is lost. Both of these effects result in reduced energy and/or position resolution and/or increased noise.
It has been and still is an object of scientists in the field to capture as much of the light as possible and to assure that the light is spread so as to illuminate a plurality of PMTs. However, it is important that the light be distributed in a manner that allows for the determination of the position of the event that caused it.
In many applications in which high energy gamma rays are utilized, the absorbance of the rays by the crystal is poor. To increase the gamma ray capture, a thicker crystal is used. However, such a thicker crystal results in reduced resolution in the image.
When the crystal scintillates, photons of light are transmitted in all directions. Accordingly, much of the light is never sensed by a PMT and is lost. This is exacerbated by the fact that the high refractive index of the crystal results in its acting as a light guide which distributes the light far from the nearby PMTs. To increase the amount of light that is sensed by the PMTs, in the prior art, reflective materials are placed on the surface of the crystal opposite to the surface coupled to the PMTs. The reflective materials capture the light that is emitted in the direction away from the PMTs and transmit it back to the PMTs. In general, this is done by reflection surfaces that are attachments to the crystal. In general, these materials may be specular or diffusive reflectors and are sometimes distributed to improve the distribution of light from events such that better resolution of the positions of the events can be maintained.
Nevertheless, the thickness of scintillator crystals is limited by resolution effects such that much of the radiation in high energy gamma imaging is lost.
U.S. Pat. No. 5,763,887 also shows reflection surfaces that are tailored to cause the light to be reflected towards the PMTs in a manner which improves the event location property of the detector. This patent describes the use of holographic reflectors or light directors that are situated on one or both sides of the crystal. Due to the difficulty in coupling light out of the surface of the crystal, this solution appears to be of limited utility.
It is known from xe2x80x9cPerformance of a position-sensitive scintillation detectorxe2x80x9d by J. S. Karp, et al. (Phys. Med. Biol. 1985, Vol. 30, No. 7, pp. 643-655, to provide, in one dimensional detectors, one dimensional surface depressions uniformly spaced on the surface of a scintillation crystal to increase the amount of light transferred to the detectors and to increase the resolution of the detector. However, despite the nearly 14 year that have passed since the publication of this paper, no practical utility in either commercial detectors or in two dimensional detectors has been made.
To overcome the faults of the light directing plates that are presently being used, one aspect of some preferred embodiments of the present invention comprises operating on the scintillation crystal per se to intrinsically provide the light direction control desired, using combinations of reflection, refraction, diffraction, or transparency. As used herein, the term xe2x80x9cintrinsicxe2x80x9d when applied to a scintillator of to functions of a scintillator, means that at least part of the function or structure referred to is part of the body or surface of the scintillator and not completely external to the outer surface of the scintillator.
Thus, some preferred embodiments of the invention provide scintillator crystals that includes intrinsic light direction controllers for optimizing the light output of the scintillator quantitatively and qualitatively. More particularly the light direction controllers include directive reflection surfaces, such as grooves, on the surfaces of the scintillation crystal, pyramids integral to the surfaces of the crystals, cones intrinsic to the surface of the crystals, or crystal surfaces for providing reflection from the crystal surfaces. The crystal surfaces are preferably shaped for directing the light by machining, etching, embossing or forging the surfaces of the crystal.
Thus, this aspect of the invention contemplates modifying the scintillation crystal itself so that it provides the desired light direction control. To this end the surfaces of the crystals are tailored so that the light illuminated areas of the detector are not directly beneath a PMT. The area directly under the PMTs is tailored to reflect light or to diffuse the light so that a greater portion of it strikes the surrounding PMTs rather than being captured for the most part by a single PMT. Thus in general the crystal is modified so that it directs the light towards the side of crystal facing the PMT and in addition directs the light to a group of PMTs rather than to single PMTs.
Furthermore, to avoid loss of light between PMTs, in some preferred embodiments of the invention, the surface of the crystal is formed to redistribute light which would have been lost between the PMTs so that the light is captured by a group of PMTs.
There are certain cases where it is desired to isolate the light so that its spread is reduced, so that it strikes only a small group of PMTs. The invention contemplates modifying the scintillator crystal in the manner which will also accomplish this isolation of the emitted light. For example, when a thick crystal is used focusing can reduce the light spread such that the spread is reduced to that of a scintillator half as thick. This allows for the use of thicker scintillator crystals (with higher capture efficiency) with the resolution of thinner crystals.
The direction of the light is controlled by machining such as by engraving or by pressing, or by embossing or even by etching the crystal to have a multiplicity of intrinsic small pyramids or rings or other forms on the surfaces thereof to direct the light as desired. Thus, for example, the pyramids can be located in defined areas and absent from other defined areas or replaced by grooves or cones in other defined areas. Thus, the pyramids can be located in a checkerboard type configuration wherein certain sections of the surfaces of the crystal have pyramids thereon other sections have simple diffraction or reflection type surfaces, and still other surfaces enable light to pass directly therethrough.
There is thus provided, in accordance with a preferred embodiment of the invention, a scintillator, having two faces, for use in medical diagnostic imaging devices, said devices including a plurality of light sensors for converting light generated in said scintillators to electrical signals, processors for converting the electrical signals to images and a monitor for displaying the images,
said scintillator including a two dimensional distribution of intrinsic light controllers, said controllers changing the light distribution among the sensors to achieve a desired distribution.
Preferably, the light controllers focus the light such that the spread of light to detectors is reduced.
Preferably, the light controllers redirect light that would strike a sensor in their absence to a farther sensor.
Preferably, the light controllers increase the light directed to the sensors.
In a preferred embodiment of the invention, the density of light controllers varies with their position on the scintillator.
In a preferred embodiment of the invention, the scintillator has a bottom face optically coupled to said light sensors and a top face spaced apart from said bottom face, and the light controllers include a plurality of angled surfaces in at least one of the faces of the scintillator for directing light that strikes the planar surfaces.
Preferably, the angled surfaces form depressions in the face. Alternatively or additionally, the angled surfaces form elevations in the face. In a preferred embodiment of the invention, the depressions and/or elevations, intersect.
In a preferred embodiment of the invention, the angled surfaces form pyramids, which direct the light impinging thereon to the sensors. In a preferred embodiment of the invention, the angles of the pyramids vary according to their position on the surface of the scintillator.
In a preferred embodiment of the invention, the angled surfaces are planar surfaces.
In a preferred embodiment of the invention, the angled surfaces form cones in the surface of the scintillator.
In a preferred embodiment of the invention, the angled surfaces form grooves.
In a preferred embodiment of the invention, at least some of the controllers are coated with reflective coating. In a preferred embodiment of the invention, the reflective coating includes tiny glass balls.
In a preferred embodiment of the invention, the controllers are formed in the face of the scintillator facing the sensors. In a preferred embodiment of the invention, the controllers are formed in the face of the scintillator facing away from the sensors.
In a preferred embodiment of the invention, the light controllers are arranged in a checkerboard fashion.
In a preferred embodiment of the invention, the intrinsic light controllers are formed directly above the sensors in the surface of the scintillator facing the sensors. Alternatively or additionally, the intrinsic light controllers are formed in a surface of the scintillator directly above regions at which no sensor is placed.
in a preferred embodiment of the invention, wherein the scintillator has dispersed therein material for absorbing scintillations.
There is further provided, in accordance with a preferred embodiment of the invention, a scintillator, having two faces, for use in medical diagnostic imaging devices, said devices including a plurality of light sensors for converting light generated in said scintillators to electrical signals, processors for converting the electrical signals to images and a monitor for displaying the images,
said scintillator having dispersed therein material for absorbing light produced by scintillations
Preferably, the scintillator has dispersed therein scatter material to scatter the scintillations.
Preferably, the edges of the scintillator are biased to reduce spread of light to sensors far from the edge.
In a preferred embodiment of the invention, the material is a colorant.
There is farther provided, in accordance with a preferred embodiment of the invention, a scintillator, having two faces, for use in medical diagnostic imaging devices, said devices including a plurality of light sensors for converting light generated in said scintillators to electrical signals, processors for converting the electrical signals to images and a monitor for displaying the images,
said scintillator having a dispersed therein material which scatters light produced by scintillations.
Preferably, the edges of the scintillator are biased to reduce spread of light to sensors far from the edge.
There is further provided, in accordance with a preferred embodiment of the invention, a scintillator, having two faces, for use in medical diagnostic imaging devices, said devices including a plurality of light sensors for converting light generated in said scintillators to electrical signals, processors for converting the electrical signals to images and a monitor for displaying the images, wherein the edges of the scintillator are biased to reduce spread of light to sensors far from the edge.
In a preferred embodiment of the invention, the scintillator includes shaped light guides which guide light from the scintillator to the sensors, the total area of the sensors being smaller than the total area of the scintillator.
There is further provided, in accordance with a preferred embodiment of the invention, a scintillator, having two faces, for use in medical diagnostic imaging devices, said devices including a plurality of light sensors for converting light generated in said scintillators to electrical signals, processors for converting the electrical signals to images and a monitor for displaying the images, and including shaped light guides which guide light from the scintillator to the sensors, the total area of the sensors being smaller than the total area of the scintillator.
Preferably, the light guides have the general shape of a truncated cone or pyramid. Preferably, the surfaces of the light guides are coated with a light reflecting material.
There are further provided, methods of forming a scintillator crystal in accordance with a preferred embodiment of the invention by machining the surface, etching the surface embossing the surface or of forging the light guids in the surface.